/*
Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        bool loghead = false;
        ListNode *prev = NULL, *cur = head;
        while (cur && cur->next) {
            ListNode *next = cur->next;
            if (loghead == false) {
                head = next;
                loghead = true;
            }
            if (prev) prev->next = next;
            cur->next = next->next;
            next->next = cur;
            // update pointers 
            prev = cur;
            cur = prev->next;
        }
        return head;
    }
};
